aes

Aes, aeris, n. Third declension neuter. It covers copper and the alloys made from it, chiefly bronze, and Latin does not distinguish the two the way modern metallurgy does.

The second sense follows from Roman economic history. Before silver coinage the state weighed out lumps of bronze, so aes came to mean money in general: aes militare is a soldier's pay, and aes alienum, literally "someone else's bronze", is the standard phrase for debt. The trap is the genitive. Aeris belongs equally to aes, bronze, and to aer, air, and only the context tells you which noun you are holding. The adjective is aeneus or aereus, of bronze, which is where English aeneous comes from.